2,6-DMN, which may be converted into naphthalene-2,6-dicarboxylic acid by oxidation, has attracted attention as an important material for the production of industrial products such as polyesters or plasticizers. 2,6-DMN is present in a petroleum fraction or a coal tar fraction in the form of a mixture with other DMN isomers. However it is extremely difficult to selectively separate 2,6-DMN from a mixture of DMN isomers by common separation procedures such as distillation, extraction, recrystallization, sublimation or adsorption, since there are ten DMN isomers which are close to each other in physical and chemical properties. Accordingly, there have been proposed some methods for separating 2,6-DMN from a mixture containing the same by forming a complex thereof together with m-nitrobenzoic acid (JP-B-47-29895 and JP-B-47-38440), p-nitrobenzoic acid (JP-B-55-44734) or other nitrobenzene derivatives (JP-B-55-47021) (The term "JP-B" as used herein means an "examined Japanese Patent publication). However these methods for separating 2,6-DMN by forming a complex thereof are disadvantageous in the selectivity for 2,6-DMN and in the separation and recovery of the same from said complex. Thus none of them has been put into practical use.